1. Field of the Invention
The present invention relates to a solar cell, more particularly relating to a thin-film solar cell and the manufacturing method thereof that make the front electrode layer and the back electrode layer at the positive electrode or the negative electrode contact directly with each other.
2. Description of the Prior Art
A solar cell is a photovoltaic semiconductor device that directly converts the energy of the sunlight into electricity and outputs a current with a voltage by the photoelectric effect. Therefore, a solar cell is also called as a photovoltaic (PV) cell. A solar cell can be classified as a silicon-based solar cell, a thin film solar cell, a dye-sensitized solar cell, or an organic/polymer solar cell according to the categories of the light-absorbing material used in the solar cell, wherein the light-absorbing material used in the organic/polymer solar cell are III-V compounds, including GaAs, InP and InGaP, and II-VI compounds, including CdTe and CuInSe2.
A solar cell mainly comprises a substrate, a front electrode layer, an absorber layer and a back electrode layer. The absorber layer can uptake incident light to generate electron-hole pairs by photovoltaic effect. Electrons and holes move toward opposing directions respectively by the intrinsic electric field built in the absorber layer. A voltage difference is therefore generated between the positive electrode and the negative electrode of the solar cell. To output a current generated between the two electrodes of the solar cell, a solder bump is disposed on each of the two electrodes. The current is therefore output from the two electrode layers via the electrical connection provided by the solder bump.
Generally, the solder bumps are extended from the back electrode layer downward by means of ultrasonic waves or vibration to contact the front electrode layer. However, in the course of setting the solder bumps, due to the poor controllability of such means in respect of output power, the resultant solder bumps are often not well located as expected, and defects such as poor contact caused by the solder bumps not deep enough to contact the front electrode layer or unsightly substrate induced by the solder bumps excessively deep to become jutting out of the substrate can happen. Moreover, the current generated by the positive electrode of the solar cell at the solder bump cannot be output via the solder bump. Therefore, the current generated there is a useless current for it cannot be utilized effectively. If the absorber there continuously generates current via photovoltaic effect, accumulation of the useless current is therefore resulted in the heat generation there and the elevation of temperature. In addition, a reverse current will be caused by the electrically serial- or parallel-contact of the positive electrode and other general electronics. The reverse current, when encounters the absorber layer that functions as an electrical resistance to it, will generate heat and temperature there will be elevated. Therefore, a conventional solar cell usually has the problem of temperature being elevated at the solder bump. If the temperature there is elevated above a threshold, the normal function of the elements of a solar cell will be effected, or even out of function. On the other hand, at the negative electrode of a conventional solar cell, the front electrode layer electrically contacts to the back electrode layer via only the solder bump there, which results a small contact area between the two electrode layers and limits the amount of the electrical current.